Bug #45058 init_available_charsets uses double checked locking

A client doing multiple mysql_library_init() and
mysql_library_end() calls over the lifetime of the process may
experience lost character set data, potentially even a
SIGSEGV.

This patch reinstates the reloading of character set data when
a mysql_library_init() is done after a mysql_library_end().

mysys/my_init.c

@@ -165,6 +165,7 @@ void my_end(int infoflag)
       my_print_open_files();
     }
   }
+  free_charsets();
   my_error_unregister_all();
   my_once_free();